Galvanometric optical scanning system having synchronization photodetectors

ABSTRACT

A galvanometric optical scanning system is disclosed. In order to compensate errors in phase, zero drift etc., at least two synchronization photodetector means are provided in an optical path of scanning beam of light. The pulses produced by the photodetector means are processed to generate correction signals for various commpensations.

FIELD OF THE INVENTION

The present invention relates to a galvanometric scanning system and more particularly to a galvanometric scanning system including synchronization photodetector means for compensating errors caused by phase variation, zero drift, gain fluctuation etc.

BACKGROUND OF THE INVENTION

Galvanometer mirrors have been widely used as light deflectors in various optical scanning devices, e.g. optical imaging devices, light projecting devices, non-impact optical printers in the art, the galvanometer mirror is angularly moved in reciprocating strokes when a sawtooth sinusoidal or other repetitive current is supplied to the mirror drive.

An excellent example of the optical scanning for imaging is found in U.S. Pat. No. 4,627,734, Dec. 9, 1986 (Rioux). In his patent, Rioux describes an optical three dimensional imaging system using a laser synchronous scanner which scans an object by a beam of light and receives synchronously scattered light from the object. By processing the received scattered light signal, a three dimensional image is reconstructed.

In U.S. Pat. No. 4,648,685, Mar. 10, 1987 Fukai et al, a specifically designed sawtooth voltage waveform is employed to scan a flat surface at variable speeds for different directions.

For data gathering, and image reproduction by optical scanning, it is necessary to synchronize the scanner to external drive electronics and to generate a pixel clock to strobe data either in or out of user hardware. Although low cost microprocessors and high speed complex logic elements have made this easier and more affordable, such synchronization with accuracy and precision is still complex.

Many patents describe variety of techniques for resolving various problems encountered in galvanometric scanners.

U.S. Pat. No. 4,127,781, Nov. 28, 1978 (Sato), for example, teaches a system for determining accurately the position of the scanning mirror. It uses a rectangular uniformily distributed light which is reflected from the back surface of the mirror across a plurality of parallel photovoltaic diode structure bars.

U.S. Pat. No. 4,044,248, Aug. 23, 1977 (Glassian), on the other hand, describes a method and an apparatus for linearizing a mirror galvanometer. The patent uses a transmission grating in the path of the beam. The grating modulates the scanned beam in accordance with the velocity of scan of the light beam. A control signal is produced by processing the output from a photocell monitoring the scanned beam and is combined with a reference driving signal to deflect the mirror in linear fashion.

OBJECTS OF THE INVENTION

It is therefore an object of the present invention to provide a galvanometric scanner which is compensated for various errors.

It is another object of the invention to provide a galvanometric scanner which is compensated for phase variations zero drift etc.

It is still another object of the present invention to provide a galvanometric scanner which include synchronization photodetector means which generates synchronization pulses for various error compensations.

SUMMARY OF THE INVENTION

Briefly stated, a galvanometric optical scanning system, according to one embodiment of the present invention includes a scanning mirror rotatably supported on an axis and a mirror drive means for driving the scanning mirror in an angularly reciprocating fashion about the axis. The system further includes a light source for projecting a beam of light towards the scanning mirror to scan an object with the deflected beam of light deflected from the scanning mirror. Galvanometer controller means is provided for applying a drive signal to the mirror drive means. There are positioned in an optical path of the deflected beam, at least two synchronization photodetector means to generate synchronization pulses when the deflected beam impinges on the photodetector means. Synchronization circuit means is connected to the synchronization photodetector means for handling the synchronization pulses and for applying correction signals to the galvanometer controller means so that the drive signal is properly compensated.

BRIEF DESCRIPTION OF THE DRAWINGS

In a more complete understanding of the present invention and for further objects and advantages, references may be made to the following description taken in conjunction with the accompanying drawings in which:

FIG. 1 is a schematic perspective view of the present invention according to one embodiment.

FIGS. 2A, 2B and 2C are graphs showing the relationships of the movement of the mirror and synchronization pulses generated.

FIGS. 3 and 4 are schematic diagrams of the galvanometer controller and the sychronization circuit means according to different embodiments.

FIGS. 5 and 6 are schematic illustrations of the present invention, according to other embodiments.

FIG. 7 is a schematic illustration of the present invention showing the arrangement of the window and the photodetector according to still another embodiment.

FIG. 8A, 8B and 8C are graphs showing the relationships of the synchronization pulses and the movement of the scanning mirror for the embodiments using two photodetectors.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1, shows a schematic perspective view of the present invention according to one of its embodiments.

A laser 1 projects a beam of light towards a scanning mirror 3 which is rotatably supported by a mirror drive means 5. The drive means is typically an electric motor or other galvanometric drive means and drives the mirror in an angularly reciprocating fashion about an axis 7 as shown by arrows 9, thereby scanning a scene 11 with a scanned deflected beam 13 along a scanning line 15. The mirror drive means 5 is energized by a drive signal sent thereto from a galvanometer controller 17. The controller 17 can produce a variety of drive signals upon command of a human operator in order to drive the scanning mirror in the desired motion e.g. sinusoidal, sawtooth etc. The embodiment shown in FIG. 1 includes a photodiode 19 at a predetermined location in an optical path of the scanned deflected beam. A transparent window 21 is provided on a housing (not shown) for passage of the scanned beam and the photodiode is provided thereon.

The photodiode 19 generates a synchronization signal in the form of pulses, when the scanned beam impinges upon it as the beam reciprocates. Therefore, the synchronization signal indicates the times when the scanning mirror is precisely at a present angle.

Known galvanometer scanners are often provided with a position sensing mechanism which produces a feedback signal to be applied to a galvanometer controller to improve frequency response of the scanners. In the above-referenced U.S. Pat. No. 4,127,781, a position determining mechanism is taught.

There are always certain errors between the drive signal and the actual position of the mirror which affect accuracy of the galvanometer. These errors are results of factors such as non-linearity, jitter, wobble, zero drift, gain drift, phase variation among others. The precise definitions of these factors are widely known and can be found in reference books.

If one measures these factors one should be able to compensate the position of the mirror precisely to be responsive to the drive signal.

The position sensing mechanism normally gives a very accurate reading on the peak-to-peak amplitude of the relative position but the arruracy of the zero is highly related to temperature.

In order to compensate these fluctuations, one needs to know with precision the phase variation φ and the zero drift (offset E_(off).

Referring back to FIG. 1, the photodiode 19 produces pulses which, after processing, can give this information with accuracy.

In order to describe the principle of the present invention, reference is made to FIGS. 2a, 2b, and 2c which show the relationships of the actual movement of the mirror and pulses (shown by bold arrows) generated by the photodiode, as the mirror reciprocates. While a sinusoidal movement is illustrated in the Figures, other waveforms, such as sawtooth are also possible.

As seen in the Figures, when the mirror oscillates, two synchronization pulses are generated in each scanning cycle.

In FIG. 2A, T_(ref) (reference value) is preset and the galvanometer controller is calibrated by a human operator. Under these conditions, the waveform shown in the figure is generated and has no zero drift. During the operation however, the zero position can shift either up or down, resulting in changes in the distance between the pulses as seen in FIGS. 2B and 2C.

The reference values T_(ref) can be set either electronically by adjusting the length of T_(ref) or by mechanically adjusting the position of the synchronization photodiode. It is also possible to arrange more than one photodiode and activate certain one to preset T_(ref).

In ideal operation, the distance T between the pulses should be always constant at a preset reference value T_(ref). Because the phase φ between input and output is small, it is always possible to distinguish one pulse from the other. If an offset (zero drift) is present as in FIGS. 2b and 2c, the distance T increases or decreases, depending upon the direction of the offset. The amount of offset is function of the distance T=(t₂ -t₁).

The phase φ is also easy to determine. As seen in the figures, the top of the sawtooth is always exactly at the center of the two pulses, thus ##EQU1##

By using these measurements, adjustments (compensation) can be effected automatically by the controller as shown in FIG. 1.

FIG. 3 shows a schematic diagram of galvanometer controller and synchronization circuit means. A waveform generator 31 generates a signal of desired waveform, e.g. sawtooth, sinusoidal ect. for driving the galvanometer drive (motor). The synchronization pulses produced by the synchronization photodiode are applied to an amplifier 33. The output of the amplifier 33 is time-delayed by preset adjustable delay means 35 which is each normally set at T_(ref) /2. The delayed output and the undelayed output are compared by a comparater 37 to produce a correction signal whose polarity and duration indicate the direction and amount of offset (zero drift). This correction signal and the drive signal generated by the waveform generator are combined to produce a corrected drive signal which is applied to the galvanometer drive.

FIG. 4 illustrates another embodiment of the galvanometer controller. In the figure, two counters 41, 43 measures t₁ and t₂ and these values are processed to generate signals representative of phase φ and correction signals.

This inventive synchronization technique is not sensitive to mechanical vibrations as the galvanometer and the photodiodes are mechanically very stable and in any case the vibrations do not influence the rotation of the scanning mirror.

The only concern may be the loss of accuracy near the end of scan when the scan changes it direction. However, this loss is of less importance because the important measurements are carried out in the central region of the scan.

The use of the synchronization pulses in the position detection according to the present invention, on the other hand, has the following further advantages over the known A/D method.

1. The technique is entirely digital and needs no special calibration and adjustment. It is also noise free.

2. The Zero position is automatically adjusted.

3. The speed of the scanning can be easily changed by a change of the clock frequency. If the sawtooth is generated by an up/down counter and a D/A converter, a programmable divider can be used for this purpose.

4. An increase in the precision of the position sensor will result in a higher resolution then would be in the A/D method.

Referring to FIG. 5, another embodiment of the present invention includes two synchronization photodiodes 51, 53 in the optical path of the scanned beam.

FIGS. 6 shows schematically still another embodiment in which two photodiodes 61, 63 are provided in the optical path of a portion of the scanned beam reflected back from a window 65. The photodiodes are positioned at locations to receive the reflected portion of the scanned beam but they do not interfere with the scanned beam.

FIG. 7 is another way of positioning the photodiode 71 out of the direct path of the scanned beam 73. A window 75 is located at an angle to reflect a portion of the scanned beam towards the photodiodes and at the same time to transmit the scanned beam towards the object.

The principle of operation of these embodiments is graphically shown in FIG. 8A, 8B and 8C in which, sawtooth waveforms for the drive signal is illustrated for the purpose of explanation. Same principle applies to other waveforms with appreciable modifications.

In these figures, P₁ and P₂ indicates the positions of the two photodiodes which measure t₁, t₂, t₃ and t₄ of synchronization pulse by producing corresponding pulses.

Now let

    T.sub.1 =t.sub.2 -t.sub.1

    T.sub.2 =t.sub.4 -t.sub.3

When a reference value T_(ref) is preset by a human operator, the galvanometer controller produces a sawtooth waveform shown in FIG. 8A. Under this condition T_(ref) =T₁ =T₂.

Then phase φ is calculated by

    φ=(t.sub.1 +t.sub.2)/2-π/2

or

φ=(t₃ +t₄)/2-3π/2

These equations can be rewritten into,

    φ=(t.sub.1 +t.sub.2 +t.sub.3 +t.sub.4)/4-π

FIGS. 8B shows a graph when an offset E_(off) is present. As seen in the figure, E_(off) is a function of T₁ and T₂. In fact, it can be expressed as

    E.sub.off ≅T.sub.1 -T.sub.2

Upon processing, E_(off) can be applied to the galvanometer controller for correction in the drive signal.

FIGS. 8C, on the other hand, shows a graph when the gain (the amplitude) of the drive signal has fluctuated. This drift in gain also can be measured and applied to the galvanometer controller for correction. It can be expressed as shown below

    Egain≅T.sub.1 +T.sub.2 -2 T.sub.ref.

It should also be noted that as in the case of the embodiment described above with reference to FIGS. 1, 2A, 2B and 2C, a pair of photodiodes can be mechanically adjusted with respect to lcoation to preset T_(ref). On the other hand, T_(ref) can be electronically preset in the circuit shown in FIGS. 3 and 4. In practice, a mechanical coarse preset and an electronic five turning are performed.

Although one or two photodiodes are disclosed in the embodiments, it is also possible to arrange more than two photodiodes so that any of the photodiodes can be activated to produce a similar effect as adjusting the locations of photodiodes. In addition to galvanometers discussed above, the technique is equally applicable to such electrically controlled devices as motors, resonant scanners etc. 

We claim:
 1. A galvanometric optical scanning system, comprising:a scanning mirror rotatably supported on an axis; mirror drive means for driving the said scanning mirror in an angularly reciprocating fashion about the said axis; a light source for projecting a beam of light towards the scanning mirror to scan an object with the deflected beam of light deflected from the said scanning mirror; galvanometer controller means for applying a drive signal to the said mirror drive means; at least two synchronization photodetector means positioned in an optical path of the deflected beam to generate synchronization pulses upon impinging of the deflected beam thereon; synchronization circuit means connected to said synchronization photodetector means for processing said synchronization pulses to generate correction signals indicative of phase variation φ, offset E_(off) and gain drift E_(gain) of the drive signal, according to the following equations: ##EQU2## where t₁, t₂, t₃ and t₄ are timings of four synchronization pulses in each scan cycle and T₁ =t₂ -t₁ T₂ =t₄ -t₃ and T_(ref) is a preset reference value; and means for applying any or all of the said correction signals to the said galvanometer controller means so that the said drive signal is compensated.
 2. The galvanometric optical scanning system, according to claim 1, wherein further comprising:an optical window positioned between the said scanning mirror and the object to reflect a portion of the deflected beam of light; and the said synchronization photodetector means positioned in an optical path of the said reflected portion of the deflected beam of light to generate synchronization pulses upon impinging of the said reflected portion thereon.
 3. The galvanometric optical scanning system, according to claim 1, wherein:the said synchronization photodetector means comprises at least a pair of photodiodes and are adjustable as to their locations.
 4. The galvanometric optical scanning system, according to claim 3, whereinthe said synchronization photodetector means comprise a plurality of photodiodes, any pair of which can be activated to adjust the locations of the said synchronization photodetector means.
 5. The galvanometric optical scanning system, according to claim 2, whereinthe said synchronization photodetector means comprises at least a pair of photodiodes and are adjustable as to their locations.
 6. The galvanometric optical scanning system, according to claim 5, wherein:the said synchronization photodetector means comprise a plurality of photodiodes, any pair of which can be activated to adjust the locations of the said synchronization photodetector means. 